According to the statistics of WHO, newly diagnosed cancer patients worldwide were as many as 12 million in 2007 and over 7 million cancer patients died every year around the world. This number was very close to the number for death from acute angiocardiopathy. Cancer is going to be the disease that leads to the most death in the world.
Nucleoside drugs, such as cytarabine, gemcitabine, decitabine, azacitidine, cladribine, fludalabine, atromide, nelarabine, 6-azauridine and tiazofurine have been widely used in the treatment for various cancers. Meanwhile many nucleoside drugs are in the clinical stage, for instance, 4′-thio-fluoro-ara-C, 2′-deoxo-2′-fluoromethylcytidine, 4′-thio-ara-C and 3′-ethynylcytidine (ECYD).
Gemcitabine hydrochloride the chemical name of which is 2-Deoxy-2,2-difluorocytidine hydrochloride is a nucleoside drug. It was developed by Eli Lilly and was approved to go on the market of Australia and Finland in 1995 with the trade name of Gemzar (Chinese trade name: Jian Ze). Gemcitabine hydrochloride is a cell cycle specific antimetabolite drug. It mainly acts on cancer cells in the DNA synthesis phase, i.e. S-phase cells. Under certain conditions, it prevents cells in G1 phase from entering into S phase. As a prodrug, gemcitabine hydrochloride is an excellent substrate of thymine deoxyriboside kinase phosphorylation. In the presence of an enzyme, it is changed to gemcitabine monophosphate (dFdCMP), gemcitabine diphosphate (dFdCDP) and gemcitabine triphosphate (dFdCTP). Both dFdCDP and dFdCTP are the active products. dFdCDP inhibits ribonucleotide reductase, thereby reduces the amount of deoxynucleotide which is necessary for DNA repair. Gemcitabine hydrochloride is effective for treating non-small cell lung cancer, pancreatic cancer, bladder cancer, breast cancer or other solid tumors. In clinics, gemcitabine has to be administrated by intravenous injection rather than oral administration due to its intestinal toxicity and low bioavailability.
5-FU (5-Fluorouracil) is a basic drug for the treatment of late-stage colorectal cancer Gemcitabine hydrochloride can facilitate binding of 5-FU to its target (thymidylate synthetase), enhancing the inhibiting effect of the latter on DNA synthesis. Colon cancer is a colon malignant epithelial tumor. It is one of the most common malignant tumors in Europe, North America and Australian. Colon cancer is the second cancer cause of death. Although the morbidity rate of colon cancer was lower in African, Asia, South America, the morbidity rate is rising as the life style is changed to western style.
However, gemcitabine hydrochloride cannot be used to treat liver cancer, because it becomes inactive quickly in liver.
Liver cancer is the third leading cancer cause of death in the world. The risk factors include HBV or HCV. It was known that the 60%-80% of Hepatocellular Carcinoma were provoked by HBV. Because of the lack of efficacious chemotherapeutic agent, there is no standard treatment for liver cancer until today. It was reported that doxorubicin was the most widely used drug, but its action was supported only by one randomized controlled trial containing 60 patients. The fatal rate of complication provoked by Doxorubicin was nearly 25%. Mitoxantrone was approved to treat Hepatocellular Carcinoma; but it is not considered as an efficacy drug for treating liver cancer.
The clinical trials results of sorafenib, the evaluation project for drugs for liver cell cancer (SHARP), reported in the 43th American Society of Clinical Oncology (2007) made a big stir. The research results indicated that sorafenib was the first drug that could prolong the life of hepatocellular carcinoma patients at late-stage. The results may change the history of the primary standard treatment for late-stage hepatocellular carcinoma.
Due to the specificity of liver function, nucleoside drugs are metabolized quickly to inactive substances in liver and lose their activities. Up to today, there is not a nucleoside drug that can be used to treat hepatocellular carcinoma. Metabasis, a U.S. company, has developed a technique of direct enzymolysis in liver, i.e. enzymatic digestion of cyclic phosphate prodrugs in liver. It has been successfully used to clinical studies of noncyclic and cyclic antiviral and anticancer nucleoside compounds. The clinical assessment of the prodrug based on cytarabine for liver cancer is in progress (US2005/0101775; Mark Eron et al., J. Am. Chem. Soc. 2004, 126, 5154-5163). However, the work of Metabasis was only limited to the cyclic phosphorylation of cytarabine on the O5 position.
The liver targeted nucleoside prodrugs have the following features:
(a). Specificity to liver, and it can be used to treat liver cancer in a targeted therapy.
(b). Accelerate the controlling steps of the phosphorylation in vivo, and increase its effect.
(c). Improved physicochemical properties, such as absorption, penetration, stability and pike performance.
(d). Reduce the side effect of nucleoside drugs without additional toxicity.
Due to common drug resistance of cancer cells and patients' urgent needs to new anti-cancer drugs, it is extremely urgent to develop new safe and reliable anti-cancer drugs from different angles to improve human health. The nucleoside prodrugs with organ specificity are one of the most promising new methods.
The nucleoside prodrugs are most promising new methods to reduce the side effect of anti-cancer drugs. The prodrugs of anti-cancer drugs are converted to active compounds when they are delivered to organ. The prodrugs of capecitabine and enocitabine etc. have been developed to overcome the defects of the nucleoside drugs. Now a lot of pharmaceutical companies are still working in developing methods for treating cancers by using other prodrugs (G. Xu, H. L. McLeod, Clin. Cancer Res., 2001, 7, 3314-3324; M. Rooseboom, J. N. M. Commandeur, N. P. E. Vermeulen, Pharmacol. Rev., 2004, 56, 53-102; W. D. Wu, J. Sigmond, G. J. Peters, R. F. Borch, J. Med. Chem. 2007, 50, 3743-3746).
The patents WO2005025552 and WO2006030217 have reported the synthesis of a series of prodrugs where the 5′-OH of gemcitabine ring was protected by carboxylic esters. Among them, there were monocarboxylic acid ester groups and biscarboxylic acid ester groups. The general formula was V-(L)n-COO-(L)m-D, V-(L)n-COO-(L)P-COO-(L)m-D, wherein D was an active pharmaceutical ingredient, n, m, p was 0 or 1. L represented C1-20 carbon chain, preferably 1-10, and more preferably 1-3. L represented (CH2)q, q was 1-3, or more than 3. V represented the terminal residue of the protect chain. V could be the groups or the atoms that were cleavable in metabolic process. Song et al. has reported that the 5′-OH of the gemcitabine ring was protected by amino acid ester groups (Song, X. et al. Mol Pharmaceutics 2004, 2, 157-167). Wu et al. has reported that the 5′-OH of the gemcitabine ring was protected by phosphate-amide groups (W. D. Wu, J. Sigmond, G. J. Peters, R. F. Borch, J. Med. Chem. 2007, 50, 3743-3746); Recently, WO2009/053654 has reported that the 5-OH of the gemcitabine ring was protected by phosphate-ester groups. The patents, WO98/32762, WO04/041203, WO2006098628 and US2007225248 have reported the simultaneous protection of the O5 and the N4 by saturated and unsaturated carboxylic esters and have conducted biological experiments.
Recently, LY2334737 a N4 prodrug of gemcitabine protected by branched chain carboxylic acid amino group has been reported by Eli Lilly (WO2006/065525, WO2007/149891, Bender, et al. J. Med. Chem. 2009, 52, 6958-6961). It is in clinical trial, but the side effect of intestine toxicity has not been fully resolved yet.
Up to today, there is no patent and literature that report the protection of the N4 position of gemcitabine by cyclodicarboxylic acid groups, including ester groups, amide groups, carboxylic groups and so on.